This invention relates to electrical capacitors and particularly to high energy density capacitors.
A high voltage, high energy density capacitor can be achieved by choosing two electrodes and a thick dielectric separating the electrodes. The thicker the dielectric, the greater the concentration of stress at the edge of the electrode. This concentration of stress becomes the limiting factor in the design of the capacitor due to dielectric failure in the high stress area.
By inserting a conductive layer idler (so-called because it is not attached to either of the terminals of the capacitor) between the two electrodes, one can form in effect two capacitors in series, one between one electrode and the idler, the other between the other electrode and the idler. Each of the two capacitors then need handle only a portion of the applied voltage. The geometry of such a configuration will reduce the edge stress significantly even though the average stress on the dielectric material remains the same. A series of many capacitances can be formed by inserting a stack of idlers between the electrodes.
In the course of manufacture, a capacitor's dielectric system may be damaged. The damage may result from local imperfections in the dielectric, irregularities in the electrode, or foreign particles which enter the capacitor winding. These damaged areas are identified when the capacitor is energized and the imperfect area breaks down. Such faults may render the capacitor unusable. The incidence of such faults is directly related to the total electrode area within the capacitor.
Capacitors wound with metallized electrodes (i.e., a dielectric such as paper or polypropylene with extremely thin coatings of a conductor such as aluminum or zinc) can be self-clearing. A fault in the capacitor can be cleared by applying the appropriate voltage to eliminate the conducting electrode at the point of the fault. This is normally done by vaporization of the electrode or conversion of the electrode from a metal to an insulating metal oxide. This will then render the capacitor usable.
Capacitors built with metallized electrodes have a limited current carrying capability. The current limit is generally associated with the connection between the metallized electrode and the external terminals. This limits the capacitor's usefulness in high energy discharge applications.